As more stringent environmental regulations enacted throughout the world, the sulfur content in fuel is demanded to a much lower level. However, the sulfur content of remained crude oil is becoming higher in the world year after year. So the development of more active ultra-deep HDS catalysts of diesel has been a challenging subject worldwide. SOx from the burning of organic sulfur-containing compounds present in fuel oils not only can cause acid rain, but also can poison irreversibly the three-way catalysts in the tail gas cleanup systems of engines. Therefore, it has been receiving much attention.
So, all countries in the world have enacted much more stringent sulfur specifications of diesel. The Chinese government will implement the sulfur specifications equal to Europe IV emission standard (<50 ppm) in Jul. 1, 2010. Big cities in China, such as Beijing and Shanghai, will advance two years to implement the standard. So, it is an urgent need for developing a new catalyst with super high HDS activity to meet ultra-deep hydrodesulfurization of diesel.
Currently, the industrial HDS catalysts have a very similar composition, in which a Group VIII metal such as Co or Ni acts as a promoter, and a Group VIB metal such as molybdenum or tungsten acts as an active component of the catalyst. The catalysts are supported on porous-alumina or silica-alumina oxide. Some of additives (such as B, Ti, P and Si) can be added to improve the catalytic performance of the catalysts. However, it is difficult to remove the sulfur-containing compounds of the alkyl substituted polycyclic aromatic thiophenes present in liquid fuels, such as dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene, with the traditional catalyst.
Furthermore, MoS2 catalyst is easy to be poisoned and deactivated in the process of hydrogenation of heavy oil. It is demanded to develop a new catalytic system to adapt ultra-deep HDS for liquid fuels and deep processing of heavy oil. Compared with the huge investment costs in the adjusting process operating conditions and building new reactors, the development of a new HDS catalyst based on the existing operating conditions which can be used in an existing production unit is a much more economical and feasible method.
It is well known for those skilled in the art that the HDS activity of traditional supported catalysts is hard to be improved by a large margin because the support itself has only limited catalytic effect, thus the activity of catalyst could only be improved by increasing the contact area between the support and reagent compounds or using the synergistic effect between the support and the active species. However, the multi-metal bulk catalyst, in other words, the unsupported multi-metal catalyst is a new generation catalyst with super high HDS activity because it has different kinds of active species and much more active sites than supported catalyst. In the recent references and patents, it has attracted much more attention due to having extremely high HDS activity.
In the U.S. Pat. Nos. 6,299,760, 6,156,695, 6,783,663, 6,712,955 and 6,758,963, the preparation methods and applications of new NiMoW unsupported catalysts were reported. It was found that this new NiMoW catalyst exhibited the HDS activity which is 3 times higher than that of a commercial catalyst. In the preparation method of the catalyst, NiMoW catalyst precursor is formed through reaction wherein ammonia was used as chelating agent, followed by calcination and sulfidation to form the final NiMoW catalyst. The preparation method had obvious disadvantages that the use of concentrated aqueous ammonia could cause pollution to the environment and that the cost for the preparation of catalyst was high.
It is obviously found from the prior art that the preparation method of the multi-metal bulk catalyst has some disadvantages: (1) The reagent is not environmental friendly; (2) The preparation cost of catalyst is relatively high; and (3) The HDS activity of catalyst needs to be further improved.